HYMENOLEPIS NANA 



325 



According to Grassi direct development takes place with omission 

 of the intermediate host, but with the retention of the larval stage ; 

 that is to say, rats infect themselves directly with Hymenolepis murina, 

 by ingesting the mature segments or oncospheres of this species, from 

 which subsequently the small larvae originate in the intestinal wall 



fee. 



c.p. 



ex. can. 



ov. 



FIG. 226. Hymenolepis nana (nl^^r^na) : cross section of proglottis from a rat. c.p,, 



cirrus sac; rec. sem., receptaculum seminis ; s.g., shell gland; ov., ovary ;/., testis ; cort. 



par., cortical parenchyma; m.l.n., main lateral nerve; ex. can., excretory canal; y-g., 

 vitellarium. (After v. Linstow.) 



sec. 



-bt.p. 



/prcav. 



FIG. 227. Hymenolepis nana: longitudinal section of an embryo, bl.p., 

 anterior opening of secondary cavity ; caud., caudal appendage ; pr. cav., primary 

 cavity; sec. cav., secondary cavity. Enlarged. (After Grassi and Rovelli.) 



(fig. 225); when fully developed they fall into the intestinal lumen 

 and become tapeworms. The identity of the two forms has neverthe- 

 less been disputed (Moniez, R. Blanchard, v. Linstow), though their 

 near relationship cannot be denied. Grassi gave mature segments of 

 Hymenolepis murina to six persons, but only one person evacuated 

 a tapeworm. This, however, proves nothing in a district where 



